We propose the development of a compact sensor system that could be used in point -of -care facilities for the early detection and monitoring of breast cancer. The sensor system is based on the ultrasensitive and specific detection of the tumor specific antigens, including CEA (carcinoembryonic antigen), MUC-l (CA15.3) and Her2/neu, using antibody-based recognition and proximity-based fluorescence resonant energy transfer (FRET). The sensor system combines the advantages of high effective binding affinities that accrue from the use of multiple binding events and low background based on the use of FRET and evanescent waveguide sensing. Two or more fluorescently labeled antibodies that recognize different epitopes of the tumor marker will be attached to the fluid upper leaf of a lipid bilayer. The lower leaf of the bilayer is attached to an optical waveguide used in transduction of the FRET signal. Antibody fragments (Fab units or single chain antibodies) will be prepared using recombinant methods and phage display. These antibody fragments will be coupled to membrane anchors (C-18 alkyl chains) that contain hydrophobic fluorescent dye molecules. The membrane anchors and hydrophobic fluorescent dyes embed into the upper leaf of the membrane. Tumor antigen/antibody binding events will bring the antibodies and fluorescent dyes into close spatial proximity inducing the FRET signal. This project will yield a compact sensor system that is as sensitive (or more sensitive) than current lab-based methods and simple enough to be used in a point -of-care facility for routine analysis of samples. Unlike lab-based antigen detection schemes, the target sensor system is fast (minutes), simple (requires a single step and no additional reagents), and inexpensive. Samples secreted from breasts onto cellulose filter pads (nipple filters) will be eluted and injected into a fluid cell that contains the active membrane on waveguides. The target fluid cell is inexpensive, readily replaceable and serves to contain the sample for disposal. In addition to providing a non-invasive approach for the early detection of breast cancer, the sensor system will provide an ideal research tool to study CEA presentation in serum and secreted samples as a function of disease progression. Although the target is breast cancer, the underlying sensor approach and the compact sensor system will be applicable to other markers (tumor specific and tumor associated antigens) for other cancers.